JVM (24) -- performance monitoring and tuning (5) -- Analyzing GC logs

1. GC Log format

1.1 review ：GC classification

Cenozoic collection ： When Eden When the area is full, the Cenozoic collection will be carried out , So the Cenozoic collection and S0 Areas and S1 Region independent

The relationship between the old generation collection and the new generation collection ： The garbage collection of the younger generation will be carried out before the collection of the older generation , Here's why ： A relatively large object cannot be put into the Cenozoic , Then it will naturally go back to the old age , If the old generation can't let go , It will first carry out a new generation of garbage collection , Then try to put... Into the Cenozoic , If you still can't put it down , For the old garbage collection , Then I put it in the old days , This is a process , Let me explain why we need to release them to the elderly , But I can't let go , The reasons for the new generation of garbage collection , This is because the new generation of garbage collection is simpler than the old generation , This saves performance

During garbage collection , The heap contains Cenozoic 、 Old age 、 Meta space / Forever ： It can be seen that Heap It includes the Cenozoic 、 Old age 、 Meta space , But when we set the heap size, we only set the new generation 、 It's just the old generation , Meta spaces are set up separately

1.1.1 What will trigger Full GC：

• There is not enough space in the old age
• Insufficient method space
• Display call System.gc()
• Minior GC The average size of data entering the old age is larger than the available memory of the old age
• Large objects go directly into the old generation , There is not enough space available in the old age

1.2 GC Classification of log

1.2.1 MinorGC

1.2.2 FullGC

1.3 GC Analysis of log structure

1.3.1 Garbage collector

1.3.2 GC Before and after

1.3.3 GC Time

1.4 Minor GC Log parsing

Log data ：

• 2020-11-20T17:19:43.265-0800： add to -XX:+PrintGCDateStamps Parameters . Log printing time Date format Such as 2013-05-04T21:53:59.234+0800
• 0.822： add to -XX:+PrintGCTimeStamps This parameter gc occurs ,Java The number of seconds since the virtual machine started
• [GC(Allocation Failure)： There was a garbage collection , This is a time Minior GC. It does not distinguish between the new generation and the old generation GC, The content in brackets is gc What happened , there Allocation Failure The reason is that there is not enough area in the Cenozoic to store the data to be allocated and fail
• [PSYoungGen:76800K->8433K(89600K)：
  • PSYoungGen： Express GC Where it happened , The area name and the used GC Collectors are closely related
    • Serial The collector ：Default New Generation Show Defnew
    • ParNew The collector ：ParNew
    • Parallel Scanvenge The collector ：PSYoung
    • The new generation is the same as the old generation , It is also related to the collector name
  • 76800K->8433K(89600K)：GC The memory area was previously used ->GC Rear cover area capacity ( The total capacity of the area )
    • If it's the new generation , The total capacity will show the total capacity of the whole Cenozoic memory 9/10, namely eden+from/to District
    • If it's an old age , The total capacity is the total memory size , unchanged
• 76800K->8449K(294400K)： Although this time is Minor GC, Only the new generation of garbage collection , But it will also print information about the total capacity in the heap . After displaying the area capacity GC After the situation , The entire heap memory area will then be displayed GC situation ：GC Used capacity of front heap memory ->GC Post heap memory capacity （ Total heap memory capacity ）, And the total capacity of heap memory = 9/10 The new generation + Old age , Then the total capacity of heap memory must be less than the initialized memory size
• ,0.0088371： Whole GC The time it takes , The unit is seconds
• [Times：user=0.02 sys=0.01,real=0.01 secs]
  • user： finger CPU Time spent working in user mode
  • sys： finger CPU Time spent working in kernel state
  • real： Refers to the total time spent in this event

1.5 Full GC Log parsing

Log data ：

• 2020-11-20T17:19:43.794-0800： add to -XX:+PrintGCDateStamps Parameter log printing . Time Date format Such as 2013-05-04T21:53:59.234+0800
• 1.351： add to -XX:+PrintGCTimeStamps This parameter .gc occurs ,Java The number of seconds since the virtual machine started
• Full GC(Metadata GCThreshold)： There was a garbage collection , This is a time Full GC. It does not distinguish between the Cenozoic GC Or the old generation GC. In parentheses is gc What happened , reason ：Metaspace There's not enough space . besides , There are two other situations that can cause Full GC, as follows ：
  • Full GC(FErgonomics)： reason ：JVM Adaptive adjustment leads to GC
  • Full GC（System）： reason ： Called System.gc() Method
• [PSYoungGen: 100082K->0K(89600K)]：
  • PSYoungGen： Express GC Where it happened , The area name and the used GC Collectors are closely related ：
    • Serial The collector ：Default New Generation Show DefNew
    • ParNew The collector ：ParNew
    • Parallel Scanvenge The collector ：PSYoungGen
    • The new generation is the same as the old generation , It is also related to the collector name
  • 10082K->0K(89600K)：GC The memory area was previously used ->GC Capacity of this area ( The total capacity of the area )
    • If it's the new generation , The total capacity will show the total capacity of the whole Cenozoic memory 9/10, namely eden+from/to District
    • If it's an old age , The total capacity is the total memory size , unchanged
• ParOldGen：32K->9638K(204800K)： The old age area did not happen GC, So this time GC yes metaspace Caused by the
• 10114K->9638K(294400K)： After displaying the area capacity GC After the situation , The entire heap memory area will then be displayed GC situation ：GC Used capacity of front heap memory ->GC Post heap memory capacity （ Total heap memory capacity ）, And the total capacity of heap memory = 9/10 The new generation + Old age , Then the total capacity of heap memory must be less than the initialized memory size
• [Meatspace:20158K->20156K(1067008K)]：metaspace GC Recycling 2K Space
• ,0.0285388： Whole GC The time it takes , The unit is seconds
• [Times：user=0.11 sys=0.00,real=0.03 secs]
  • user： finger CPU Time spent working in user mode
  • sys： finger CPU Time spent working in kernel state
  • real： Refers to the total time spent in this event

2. GC Log analysis tool

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